1. Field of the Invention
The present invention relates to an optical pickup device. More particularly, the present invention relates to an optical pickup device being provided with a position-controllable collimating lens and a dedicated optical element for compensation of aberration.
2. Description of the Related Art
Optical recording media currently used include a compact disc (CD) and digital a versatile disc (DVD). Recently, blue-ray discs (BD) using a blue ray have been marketed. The emergence of new optical recording media is intended for high-density recordation which can be implemented by shortening a wavelength of a light source or increasing the numerical aperture (NA) of an object lens. Although a wavelength of 650 nm, and a NA of 0.6 are used for DVD, use of a blue light source having a wavelength of 400˜408 nm and increased NA of an object lens up to 0.65˜0.85 may be considered for a higher-density optical disk.
However, some problems arise when using a short wavelength of a laser light source or higher NA of an object lens. Such problems which were almost ignorable in an optical pick-up device comprising a combination of a low NA object lens and a comparatively long wavelength of a laser light source for implementing recordation and reproduction of information relative to the conventional optical disc such as CD or DVD must be addressed in a successful implementation of a BD device.
One of the problems is an axial chromatic aberration occurring in an object lens due to minor oscillating wavelength changes in the laser light source. Change of refractive index due to the minor wavelength changes in general optical lens component increases as the wavelength is shorter. Accordingly, the amount of defocusing of a focus generated due to the fine wavelength change is increased.
However, the shorter the wavelength being used in the object lens, the lower a depth of the focus is. As a result, even a small amount of defocusing is not allowed. Therefore, in an optical system utilizing a short-wavelength light source such as a celadon semiconductor laser and a high-NA object lens, correction of chromatic aberration becomes a critical matter in order to prevent mode hopping of the semiconductor laser, change in wavelength resulted from change of output power, or deterioration of wave front aberration due to overlapping at high frequency.
Another problem caused by a high NA object lens and a short wavelength of the laser light source occurs when using a dual-layer optical disc comprising two data layers. In general dual-layer optical discs, more specifically in the BD, an interval between first and second layers is approximately 25±7 μm, whereas the interval between first and second layers in-the high definition (HD)-DVD is approximately 20˜40 μm. In order to record and reproduce data with respect to each layer, the laser light needs to be focused on the layer in recording or reproducing.
In the dual-layer optical disc, however, lengths of optical paths for the laser light being incident to the optical disc to advance to the respective layers are different. Accordingly, a range of spherical aberration increases in the dual-layer optical disc, compared to in the conventional single-layer DVD or dual-layer DVD in which the spherical aberration is within a predetermined error range.
The optical system under such an environment may have a collimating lens configured by a diffractive optical element (DOE) pattern so as to correct the chromatic aberration.
On the other hand, the chromatic aberration may be corrected by constructing an afocal optical system by a concave lens and a convex lens and adjusting an interval between the lenses.
Although the former optical system utilizing the DOE is widely used, it has a problem in that a coma aberration is abruptly generated as an incident angle of the light increases since the aberration changes according to a variation of the incident angle of the light. The coma aberration refers to a state that an image is not properly formed because of the light being obliquely incident.
The latter optical system using two lenses for correction of chromatic aberration also has a problem of its increased volume. Also, since the two lenses move together, a margin of tolerance decreases in producing the optical system, thereby degrading productivity and increasing the manufacturing cost of the optical pickup device.
Furthermore, recently, an improved lens capable of being compatibly used in both the HD-DVD and the BD has been in need. In this case, however, since thickness of the discs used in the HD-DVD and the BD are respectively 0.6 mm and 0.1 mm, spherical aberration becomes considerable.